Pyroclastic-flow deposit erupted byMount Mazama, Crater Lake, Oregon

Photograph by S.R. Brantley in
July 1984

These pyroclastic-flow deposits of the climactic eruption of Mount Mazama, during
which Crater Lake caldera formed, are eroded into spectacular spires at The
Pinnacles on Wheeler Creek. The deposit, here about 100 m thick,
accumulated as a stream of hot pyroclastic flows carrying melon-sized
pumice blocks and fine ash particles descended from Mount Mazama over the
course of a few hours about 7,700 years ago.

The magma chamber that supplied the eruption was zoned in composition
from silica-rich rhyodacite down into more iron- and magnesium-rich
andesite. This resulted in the first-erupted and deposited material being light colored
and later-erupted magma being dark. In other words, the color layering in the
pyroclastic-flow deposit can be thought of as an inverted view of the
pre-eruption layering in the magma chamber below where Crater Lake caldera
collapsed.

In detail, the pumice blocks in the deposit, which represent
samples of frothed magma, are virtually all rhyodacite in the bottom third
of the exposure. The pumice blocks are a mixed population of rhyodacite and andesite in the
middle third, and are dominantly andesite (and some very crystal-rich, more
mafic compositions) in the top third. The fine ash matrix that is
responsible for the gross color of the deposit darkens upward partly owing
to composition and partly to greater heat retention of the later-emplaced
material as the eruption was waning. The retention of heat resulted in crystallization of tiny
iron oxide mineral grains that impart a gray color to the volcanic ash.

As the deposit cooled, gases escaped from within it in open fractures
that narrowed into tubes upward, resulting in growth of new gas-deposited
minerals in the pore spaces between particles. This cemented "tuff" is
more resistant to erosion by water and wind than the surrounding deposit so
that, in time, the pinnacles stand out as spires. The red band at the top
of the bluff is fine ash in which the iron oxidized in contact with the
atmosphere as the deposit cooled.

For many years after the climactic eruption, the valleys around
Mazama must have resembled Alaska's Valley of Ten Thousand Smokes.
The "smokes" were fumaroles venting on the surface of the
pyroclastic-flow deposit, now marked in Wheeler Creek by The Pinnacles.